1. Field of the Invention
The present invention relates to a device for detecting and eliminating leakage of a working fluid from a hydraulic accumulator, through a control valve which has an open and a closed position for fluid communication and disconnection between the accumulator and an actuator to be operated by the pressure of the fluid.
2. Discussion of the Related Art
A leakage flow of a working fluid from a control valve causes a problem in a hydraulically-operated brake system for a motor vehicle as disclosed in laid open Publication No. 3-213701 of unexamined Japanese Patent Application 2-5736. This hydraulic brake system, which is adapted to be used for a four-wheel motor vehicle, has an anti-lock brake pressure regulating function and a wheel traction control function. The brake system is equipped with an accumulator storage pressure sensor for holding the storage pressure of an accumulator (hereinafter referred to simply as "accumulator pressure") within an optimum range, and an accumulator alarm pressure sensor for detecting an abnormal drop of the accumulator pressure and providing an alarm. When the accumulator pressure falls below the lower limit of the optimum range due to a supply flow of the fluid from the accumulator, the accumulator storage pressure sensor generates an output signal to activate a hydraulic pump for raising the accumulator pressure up to the upper limit of the optimum range. In the event that the hydraulic pump fails to operate in response to the output signal of the accumulator storage pressure sensor, for some reason or other, the accumulator alarm pressure sensor generates an output signal to provide a suitable alarm when the accumulator pressure falls below a predetermined reference level, which indicates an abnormal drop of the accumulator pressure.
In the hydraulic brake system described above, a solenoid-operated directional control valve is provided between the accumulator, and the actuator in the form of a wheel brake cylinder for applying brake to a vehicle wheel. This directional control valve is used for the brake system to effect the traction control of the relevant wheel. The directional control valve has a first position for fluid communication between the wheel brake cylinder and the accumulator while disconnecting the wheel brake cylinder from the reservoir, and a second position for fluid communication between the wheel brake cylinder and the reservoir while disconnecting the cylinder from the accumulator. During use of the directional control valve, foreign matters may be caught between the valving member and the valve seat, preventing the valving member from being correctly seated on the valve seat for establishing the second position. In this event, the working fluid may leak from the accumulator into the reservoir through the directional control valve, even when the valve is commanded to be placed in the second position for disconnection of the wheel brake cylinder from the accumulator. In this case, too, the accumulator pressure is regulated according to the output signal of the accumulator storage pressure sensor, and an alarm is constituted based on the output signal of the accumulator alarm pressure sensor.
However, the hydraulically operated brake system of the type described above is not adapted to eliminate the trouble with the solenoid-operated directional control valve, that is, not equipped with means for eliminating the incomplete disconnection of the accumulator from the reservoir by the valve. Consequently, once the trouble happens with the directional control valve, the fluid continues to leak into the reservoir through the directional control valve, until the valve is operated again. In the presence of the continuing fluid leakage, it takes a considerably long time for the pump to raise the accumulator pressure to within the optimum range, and the accumulator pressure tends to be rapidly lowered below the lower limit.
The above-indicated defect of the solenoid-operated directional control valve results in increasing the energization time of the pump motor and the associated relay, or the number of energization of these electric components, leading to shortening the life expectancy of the electric components, and wasting of the electric energy.
The above problem exists not only in the hydraulically-operated brake system of the type indicated above, but also in a hydraulically operated system in general, which employs a control valve such as a directional control valve or a shut-off valve, and a hydraulic accumulator for storing a working fluid under pressure.